It was a Rifle No. 4, Mk 1, made at Long Branch in Canada during WWII. It is not any kind of rare experimental.

It is now a hunting rifle. If you want a No. 4, that one could be restored, but No. 4's are still inexpensive, so balance the cost of the necessary parts against the cost of an unmodified rifle. Don't forget 1) the hassle of finding all the right parts and the time involved in doing the work and 2) the restored rifle won't be at all "correct" as the parts will be a mishmash.